Medical devices have been provided for treating benign prostatic hyperplasia by the use of radio frequency energy. See, for example, U.S. Pat. Nos. 5,370,675, 5,421,819 and 5,549,644. Radio frequency energy passing from an electrode of such a device through the adjoining tissue causes heating of the tissue. In order to cause tissue ablation and subsequent necrosis, the treated tissue is heated to a temperature in excess of 47° C.
Radio frequency generators can be provided with power levels up to several hundred watts for accomplishing such ablation and necrosis. Unfortunately, the amount of power that can practically be delivered to a patient is limited by physiological factors. For example, when a patient is unconscious under a general anesthetic, a few hundred watts of radio frequency power can be delivered for short periods of time. The amount of power that can be delivered to a conscious patient is under a hundred watts.
Some of the previously provided medical devices permit a liquid to be introduced into an area adjacent a conductive electrode. See in this regard U.S. Pat. Nos. 5,370,675 and 5,421,819. Other devices have been provided for introducing a conductive liquid through an electrode into tissue to be ablated. See, for example, U.S. Pat. No. 6,016,809. Prior art lesion producing devices utilizing an infused conductive liquid have failed to recognize the importance of maintaining a specific range of current density distributed over the effective electrode area.
It would be desirable to produce a predictable necrotic lesion with a minimum amount of power.